1. Field of the Invention
The present invention relates to an optical scanning system and, more particularly, to a post-objective type optical scanner in which a laser beam converted into a pencil of rays is deflected by a polygonal mirror. Still more particularly, the present invention is concerned with a post-objective type optical scanner which utilizes an anamorphic single lens.
2. Description of the Related Art
Post-objective type optical scanners employ a laser beam source which emits a pencil of rays and a polygonal mirror which deflects the pencil of rays. Because of compact construction and low costs of production, this type of optical scanner has been studied, developed and put to commercial production in recent years.
In general, an optical scanner has a main scanning optical system which conducts scanning in the direction of the major axis of an elliptical cross-section of a beam from a laser diode, and a sub-scanning optical system which performs scanning in the direction of the minor axis of the elliptical cross-section.
In a post-objective type optical scanner of the present invention, the focal distance of the main scanning optical system is more than 10 times as large as that of the sub-scanning optical system. Therefore, in order to equalize the dimensions of the beam spot on the image surface both in the directions of main scan and sub-scan, it is necessary that the aperture stop disposed on the incident side of the polygonal mirror has a rectangular or an elliptical form which is 10:1 or greater in terms of the ratio between the width as measured in the direction of main scan and the width as measured in the direction of sub-scan. For this reason, a mere collimation of a semiconductor laser beam does not provide a required level of rate of utilization of light. A solution to this problem is to use prisms. FIG. 5 illustrates an optical scanner which uses prisms. This optical scanner has a semiconductor laser 41, a collimator lens 42, prisms 43, 44, an aperture stop 45, a convergent lens 46, a cylindrical lens 47, a mirror 48, a polygonal mirror 49 having cylindrical surfaces, a compensating lens 50, and a photosensitive drum 51. As will be seen from FIGS. 6a and 6b, the laser beam from the semiconductor laser 41, which is disposed such that the direction of greatest divergence angle of beam coincides with the direction of main scan, is changed into a collimated beam having an elliptical intensity distribution of about 3:1 in terms of the ratio between the size in the direction of main scan and the size in the direction of sub-scan. The beam is then transmitted through prisms 43, 44 which are arranged to contract the beam spot size only in the direction of sub-scan, so that the beam is changed into a collimated beam having an elliptical form of intensity distribution of about 10:1 in terms of the ratio between major and minor-axes. The beam then impinges upon the cylindrical lens 47 through the aperture stop 45 and the convergent lens 46.
A lens which plays the double role of a collimator lens and prisms, intended for use in optical disk systems, has been disclosed in Japanese Patent Unexamined Publication No. 61-254915.
The use of prisms, however, is disadvantageous in that the cost is raised and the size of the system inevitably is large due to intricacy of arrangement of optical components. The lens shown in Japanese Patent Unexamined Publication No. 61-254915 also has drawbacks in that it exhibits relatively large residual spherical aberration because its surfaces are toric surfaces which are represented simply by radii of curvature, and in that this lens does not offer any advantage when applied to a post-objective type optical scanner since it is a lens intended for changing a beam having a flattened cross-section into a circular cross-section.